GWTGuided Wave Testing is another term for long-range ultrasonic testing and is a method for the rapid screening of pipework for corrosion and erosion. A description of long-range ultrasonic testing, or GWT, was given in the March 2008 What the Hec?!, but it is worth highlighting the GWT acronym because this is the term used in the new range of standards that have been, and are due to be, issued.

The standards give the definition of guided waves (GW) as sonic or ultrasonic waves that travel along an object and are guided by its surfaces or shape, and whose wavelength is large compared to a characteristic dimension such as wall thickness. GWT is the testing of a large volume of material from a test location utilising the long-distance propagation characteristics of guided waves.

There are different kinds of guided waves and in a pipe these modes can involve circumferential motion as well as axial motion. The different ‘orders’ of mode relate to the extent of the circumferential motion, with zero order having no circumferential motion, first order having one cycle of a sine wave around the circumference, etc. There are three families of modes:
  • Torsional (T), which have only displacements in the circumferential direction. These are zero order modes.
  • Longitudinal (L), which have displacements in the axial and radial directions only and so are also zero order modes. These displacements are referred to as extensional and breathing motions.
  • Flexural modes (F), which have displacements in all directions giving a bending motion. These flexural modes cover all the other higher order modes.

The labelling of the specific modes follows the notation T(0,1), where the letter is T, L or F, the first number in the brackets is the circumferential order and the second number is a sequential counting number.

For information on GWT see:
BS 9690-1:2011 Non-destructive testing – Guided wave testing, Part 1: General guidance and principles
BS 9690-2:2011 Non-destructive testing – Guided wave testing, Part 2: Basic requirements for guided wave testing of pipes, pipelines and structural tubulars

Keep an eye out for the following, which are in preparation:
BS 9690-3:2011 Non-destructive testing – Guided wave testing, Part 3: Training and certification requirements
BS 9690-4:2011 Non-destructive testing – Guided wave testing, Part 4: Interpretation and reporting

What the hec?! articles are not intended to be the definitive account on the topic or acronym in question. Readers’ comments and contributions are welcomed. Email: